There are generally three methods of transferring energy wirelessly. The first method transfers power using electromagnetic induction, the second method transfers the power using wireless radio frequency, and the third method transfers the power using ultrasonic wave.
The first method of transferring the power using the electromagnetic induction is disclosed in US Patent Application Laid-Open No. 2008/0265835 (Wireless Power Transfer System, Daniel P Reed) and Korean Patent Application No. 2007-0092435 (System for Wireless Charging, LG Electronics Co., Ltd, Kim Sung Il). The method includes a charging body generating charging power by using an external power supply and a power receiving module receiving the charging power from the charging body through an electromagnetic induction. The electromagnetic induction method was almost commercialized because of the high efficiency.
However, an electromagnetic wave is limited to be used within a close distance ranging from several centimeters from the charging body because transfer energy is rapidly reduced to be in inverse proportion to squared distance according to a distance in air. In order to increase an operational distance, a size of an element is increased and for example, in order to transmit the electromagnetic wave in high efficiency at a transmitted distance of 1 m, the element has the size of about 1 m. In addition, since the charging power generating device needs large power, a problem on the harmfulness to the human body is present around the power generating device.
The second method of transferring the power using the wireless frequency is disclosed in US Patent Application Laid-Open No. 2007-0109121 (Harvesting Ambient Radio Frequency Electromagnetic Energy for Powering wireless Electronic Device, Sensors and Sensor Networks and Applications Thereof, Marc H. Cohen). In this disclosure, energy of an RF having a very long wave distance is collected, so as to be used in an electronic device, a sensor, and a sensor network. The RF exists much even in air and the wave distance thereof is very wide, but the energy density is very low and the energy amount is very small even after energy conversion, such that actual efficiency is deteriorated.
The third method of transferring the power using the ultrasonic wave is disclosed in U.S. Pat. No. 6,798,716 (System and Method for Wireless Electrical power Transmission, Arthur Charych, BC System Inc.). The disclosure discloses a method of collecting a beam in the center by arranging ultrasonic wave generating devices and controlling phases thereof and a method of controlling a beam direction through a phase control.
In addition, US Patent Application Laid-Open No. 2010/0164433 (Wireless Battery Charging Systems, Battery Systems and Charging Apparatus, Anand Janefalkar, Motorola Inc.) discloses a method of charging a portable terminal mounting a system of charging a battery using an ultrasonic wave generating device.
However, in the related art, the method of transferring the power using the ultrasonic wave collects the ultrasonic wave by controlling the phase and the direction of the ultrasonic wave, while does rarely not disclose that the effective area of the radiation plate in the ultrasonic wave generating device is controlled to maximize the energy transfer efficiency, like the present disclosure.